1. Field of the Invention
The present invention relates to a semiconductor memory device and a method of fabricating the same. More particularly, this invention pertains to the improvement of a fuse of a redundancy circuit (defect sparing circuit) included in a semiconductor memory device, and a method of fabricating a semiconductor memory device having the improved fuse.
2. Description of the Related Art
Recently, semiconductor memory devices have been designed to have an ever increasing memory capacity. The large-scale increase in the memory capacity increases the number of memory cells one chip holds. It is therefore very difficult to have no defective cells in a single chip. Today, therefore, the redundancy technique to replace defective memory cells by redundant memory cells is essential in fabrication of semiconductor memory devices.
A redundancy circuit should have a program element which stores redundancy information to replace defective memory cells with spare memory cells. A polysilicon fuse is normally used as the program element, and redundancy information is stored by cutting the fuse by means of a laser.
Of semiconductor memory devices, some EPROMs, a non-volatile type, may use an EPROM memory cell as a program element with its top covered with an aluminum film to prevent the stored information from dischanging at the time of irradiating ultraviolet rays for data erasure.
These techniques however have the following short-comings.
The former technique can cut the fuse only before packaging and cannot spare defective memory cells after packaging.
The latter technique should cover the top of each program element with aluminum or the like to inhibit irradiation of ultraviolet rays or the like on the program element. The aluminum film for shielding the ultraviolet rays or the like needs a very large area in one chip. In addition, the ultraviolet rays are likely to reach this type of program element through reflection from various portions even if the top of the program element is coated with the aluminum film, so that the reliability of the program element holding data is not sufficient. Further, the data holding characteristic of the program element is quite the same as that of an ordinary EPROM memory cell. When an acceleration test (leaving a device for a long period of time under a high-temperature environment) is conducted to check the data-retention characteristic of an EPROM cell, therefore, electrons accumulated in a floating gate may leave, resulting in malfunction of the device.
As described above, the conventional program elements cannot spare defective memory cells after a chip is sealed in a package. Also the conventional program elements have a lower reliability of holding data though they occupy considerable space in a single chip.